Firebrand transport is studied for disc and cylindrical firebrands
by modelling their trajectories with a coupled-physics fire model,
HIGRAD/FIRETEC. Through HIGRAD/FIRETEC simulations, the size
of possible firebrands and travelled distances are analysed to
assess spot ignition hazard. Trajectories modelled with and without
the assumption that the firebrands' relative velocities always
equal their terminal velocities are. Various models for the flight
and combustion of disc- and cylindrical-shaped firebrands are
evaluated. Eight simulations are performed with surface fuel
fires and four simulations are performed with combined surface
and canopy fuels. Firebrand trajectories without terminal velocity
are larger than those from models with terminal velocity. Discs
travel further than cylinders, as discs are aerodynamically more
favourable. Thin discs burning on their faces and tall cylinders
burning around their circumference have shorter lifetimes than
thin discs burning from their circumference or longer cylinders
burning from their ends. Firebrands from canopy fires, with larger
size and potential to ignite recipient fuel, travel further than
firebrands from surface fires. In the simulations, which included
a line fire ignition in homogeneous fuels on flat terrain, the
firebrand launching patterns are very heterogeneous, and the
trajectories and landing patterns are dominated by the coupled
fire-atmosphere behaviour.

Keywords: coupled-physics fire model, spotting, WUI
fires

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